1. Field of Invention
The present invention relates to a microwave sensor (hereinafter “MW sensor”). In particular, the present invention pertains to a strategy for achieving improved reliability with MW sensor(s).
2. Conventional Art
Conventionally known as one type of security device is an MW sensor wherein microwaves are transmitted toward a protected area, and in the event that a person (intruder) is present within the protected area, waves reflected from that person (microwaves modulated due to the Doppler effect) are received and the person is detected.
Moreover, also known as one type of MW sensor 1 is a device which employs a plurality of microwaves of different frequencies and which is constituted so as to permit measurement of the distance to a person or other such object (hereinafter “target”) present within a protected area. In this type of sensor, microwaves of, for example, two different frequencies are transmitted toward a protected area, and the difference in phase between two IF signals based on the respective reflected waves is detected. This phase difference is correlated to distance to the target, the phase difference tending to increase with increasing distance to target. In other words, the distance to the target can be measured by calculating this phase difference. Furthermore, by identifying the temporal change in this phase difference, it is also possible to determine whether the target within the protected area is moving. This makes it possible to achieve a situation whereby, for example, only moving targets within the protected area are determined to be targets (intruders) which should be detected. Below, operations for detection of phase difference between/among IF signals in this type of sensor are described.
Taking the case where IF signals based on waves produced by reflection of microwaves of two different frequencies are sinusoidal waves IFout1, IFout2 (having phase difference corresponding to distance to target) as shown at FIG. 3(a), rectangular waves A, B derived from these IF signals might respectively be as shown at FIG. 3(b). It will, moreover, be possible to measure the distance to the target by detecting the phase difference between these rectangular waves A, B (the difference in phase Δt of the rising edge at the respective rectangular waves in the drawing). Furthermore, by identifying the temporal change in the phase difference between these rectangular waves A, B, it will be possible to identify movement of the target (whether it is approaching the sensor or receding from the sensor) within the protected area.
However, when this type of sensor is used as a security sensor and the temporal change in the aforementioned phase difference is identified so that only targets moving within the protected area are determined to be targets (intruders) for detection, it is believed that there is room for improvement with respect to the following points.
In the event that this type of sensor is installed outdoors, swaying of plant life or the like due to wind can cause a phase difference to occur between the foregoing rectangular waves A, B, as a result of which it is possible that plant life or the like might be mistakenly detected as the target which is intended to be detected, resulting in generation of a false alarm.
Similarly, in the event that this type of sensor is installed indoors, rotation of fans for circulation of air or swaying of curtains or blinds due to wind can also cause a phase difference to occur between the foregoing rectangular waves A, B, as a result of which it is possible here as well that an object other than a person might be mistakenly detected as the target which is intended to be detected, resulting in generation of a false alarm.
There is accordingly a need to accurately distinguish between persons or other such targets intended to be detected and objects not so intended (plant life, fans, etc.), and to prevent false alarms.
One possible improvement addressing this point involves measuring the amount of change per unit time in the relative distance to the target within the protected area based on respective reflected waves, and causing determination to be made that the target is the target which is intended to be detected only when this amount of change is greater than or equal to some prescribed amount. In other words, in contrast to the small distances moved by plant life swaying due to wind, rotating fans, and the like, the distances moved by persons or other such objects which are intended to be detected are large. Identification of this difference makes it possible to determine whether a target is that which is intended to be detected or not.
However, the present inventor has recognized that further improvement is necessary in order to permit more accurate determination.
In accordance with the aforementioned proposal, where a target engages in a repetitive pattern of moving by a small amount and then stopping, and thereafter again moving by a small amount and then stopping, as is the case with the movement of person A shown in FIG. 2, the change in relative distance per unit time from MW sensor 1 to target A will be small, thereby preventing this target (person) A from being determined to be a target intended for detection.
The same sort of situation also exists where the target gradually advances toward MW sensor 1 while moving to the left and right, as is the case with the movement of person B shown in FIG. 2. That is, in such a case, despite the large amount of movement of target B, because the relative distance between target B and MW sensor 1 only decreases by a small amount at a time, the change in relative distance per unit time from MW sensor 1 to target B is likewise small, preventing this target (person) B from being determined to be a target intended for detection.
Furthermore, one possible means for avoiding such undetected intrusion events is to previously set the change in the amount of relative distance per unit time necessary to achieve a determination that an object is a target which should be detected (i.e., the trigger value) to a low value so as to cause generation of an alarm even where movement of a target within the protected area is small.
However, in the event of presence of the aforementioned plant life swaying due to wind, rotating fans, and the like, the foregoing stratagem will cause these to be determined to be targets intended for detection, resulting in false alarms.